Methomyl oxime is a commercial product; is useful in the production of certain commercial insecticides of the carbamate and methyomyl type.
There are numerous references in the literature to the preparation of this economically useful intermediate and related alkyl thiolhyroxamates. There can be mentioned Israel Pat. No. 26,830, U.S. Pat. Nos. 3,658,869, 3,752,841 and 3,787,470. Most of these prior art references are characterized by either low yields or high reaction dilutions, where the low yields are due to the highly reactive nature of the reaction environment.
To overcome this critical environmental factor, many of the prior art synthesis have been carried out at great dilution and using a specific solvent. Thus, U.S. Pat. No. 3,987,096 discloses an aqueous process which gives yields of about 99% of methomyl oxime; but at a reaction concentration of less than 1%. Such a low concentration is particularly wasteful of energy in the steps required for economic recovery of the valuable intermediate or its ultimate products.
The yields of a similar magnitude of methomyl oxime was reported in U.S. Pat. No. 3,752,841 using diemthylformamide (DMF) as a reaction solvent. While this solvent permits reaction concentrations of 20-25%, DMF is a recognized irritant and toxicant substance requiring stringent precautions in the workplace under current safety legislation.
Recently U.S. Pat. No. 4,327,033 disclosed the preparation of methomyl oxime in good yield using N-methylpyrolidone. However, this also limits the reaction to one specific expensive, high boiling solvent which is difficult to remove.
U.S. Pat. No. 3,374,260 discloses the preparation of methomyl oxime by reacting S-alkyl thiolimidic ester salts with a hydroxylamine salt in the presence of metal alkoxides using a lower alkyl alcohol as solvent. However, this reaction requires six or more hours for completion, is limited only to specific types of alcohols, requires the use of the expensive and difficult to handle metal alkoxides, and affords low yields. Indeed, in attempting to reproduce the results of this patent the product was obtained in very low yields due to its decomposition under the basic reaction conditions. In addition, this reaction was found to have the tendency to get easily out of control and even to cause a slight explosion.